This invention relates to an optical recording technique, and more particularly to a phase change optical recording medium in which a recording layer is formed on a substrate whose optical characteristic changes with crystal and amorphous phase change.
A so-called "phase change optical recording medium" is an example of an optical recording medium in which the recorded data can be rewritten. The phase change optical recording medium is based on a crystal and amorphous phase change. The phase change optical recording medium is used as follows: A laser beam having a diameter of the order of one micron is applied to the recording layer of the recording medium, and the recording layer's part thus irradiated with the laser beam is made amorphous by melting, and by cooling it quickly, to record data. On the other hand, an intermediate power laser bream is applied to the recording layer to hold the recording layer's part thus irradiated at a temperature higher than the crystallization temperature for a predetermined period of time, thereby to erase the data. Reproduction of the data is achieved as follows: A low power laser beam is applied to the recording layer, to detect the difference in optical characteristic between crystal and amorphous parts as the difference in the quantity of light between light beams reflected therefrom, thereby to reproduce the data. By modifying the intensity of the laser beam with high power and intermediate power according to data, current data can be written over the previous data.
In the phase change optical recording medium of this type, in general, protective layers are provided on the upper and lower surfaces of the recording layer, respectively, which is formed on a substrate. Of those protective layers, the one provided on the light incident side (hereinafter referred to as "a light incident side protective layer", when applicable) should function as follows: That is, it is essential for the light incident side protective layer to have a physical function to prevent the recording layer from being thermally deformed when data are written or rewritten with the laser beam or when the record is erased, and an optical function to increase the interference effect to thereby improve the CN ratio and recording sensitivity (cf. Published Unexamined Japanese Patent Application (OPI) No's 195537/1990 and 180945/1986). Heretofore, in order to give the above-described optical function to the light incident side protective layer, the protective layer material is made larger in refractive index than the substrate material, and the interference effect is utilized which is provided by the protective layer formed with the protective layer material larger in refractive index. Thus, it is essential that, in the phase change optical recording medium, the light incident side protective layer is larger in refractive index than at least the substrate. Thus, while the refractive index (n) of the substrate material is of the order of 1.45 to 1.6, the light incident side protective layer is made of a high refractive index material having a refractive index of 2.0 or higher, such as ZnS-SiO.sub.2 (n =2.0) or ZnS (n =2.3).
The above-described phase change optical recording medium suffers from the following difficulties; When it is used for a recording operation, it is necessary to heat the recording material at a high temperature, higher than about 600.degree. C. Thus, the recording operation requires a large quantity of energy; that is, the recording sensitivity is low, and the number of times of rewriting operations allowed is small.
In order to eliminate the above-described difficulties, a method has been proposed in which the thickness of the recording layer is decreased to reduce its thermal capacity, thereby to decrease the amount of energy required for writing, rewriting, or erasing data (cf. Published Unexamined Japanese Patent Application (OPI) No's 195537/1990 and 195538/1990). Since the amount of energy required for a recording operation is decreased in the above-described manner, the following merits are obtained: When it is assumed that the optical absorption coefficient is the same, then the recording sensitivity is increased, the thermal load of the optical recording medium is decreased, the thermal deformation of the recording layer is decreased, and the number of times of rewriting operations is increase. (T. Ohta et al. SPIE, vol 1078 ODSTM, p 27, 1989).
However, in the phase change optical recording medium, it is not allowed to reduce the thickness of the recording layer to less than a certain value. If, in the case where the material of the recording layer is of a Ge-Sb-Te group, the thickness of the recording layer is reduced to smaller than 20 nm as shown in FIG. 8, then the difference in reflectivity between crystal and amorphous parts in the recording layer is decreased, and the CN ratio is not large enough, and the reflectivity of the whole recording medium is decreased, with the result that the servo operation such as a focusing servo, a tracking servo and the like becomes unstable. Thus, it is limited to some extent to decrease the thickness of the recording layer to improve the recording sensitivity and to increase the number of times of rewriting operations.